1. Field of the Invention
This invention is directed at a system to provide commutation control of voltages for brushless DC motors.
2. Background Art
There are methods of electronic commutation of DC motors that time-sample the back-EMF voltage of the undriven leg of a "STAR" configuration motor (a motor which uses a center tap for supply voltage connection). Instantaneous voltage samples relate to rotor angular position. Other methods periodically cut motor current and sample the voltage across all the motor windings. The angular position of the rotor is deduced from the value of these voltages. Still other methods cut motor current and then pulse current through the various motor windings to measure the rotor inductance. The angular position of the rotor relates to rotor inductance. In all of these methods, commutation is linked to the rotor position by translating rotor position to the motor drive signals required to advance the motor into the next commutation state.
The "STAR" motor configuration samples the back-EMF across the undriven leg of a motor employing unipolar drive. This method is inefficient and has other disadvantages. The sampled leg is undriven and is wasted in terms of motor drive. The bipolar motor could be driven as two motor legs in opposite polarities, whereas the "STAR" method is unipolar and only driven in one direction. Further, time sampling of the back-EMF is employed, which may require the continuing use of a microprocessor and extensive hardware. The microprocessor is an expensive device which increases the overall commutator cost. The sampling methods are also dependent on the use of inductive devices to sense current. These sensing devices are also expensive, and usually cannot be made on a monolithic integrated circuit.
Methods that turn off power to measure back-EMF suffer from reduced motor efficiency due to the loss of torque while the measurement takes place. No electrical force can be applied while the sampling is in progress. This inefficiency increases with motor speed. Also, methods which switch motor currents are noisy, both acoustically and electrically.
Prior art designs often require a controller, such as a microprocessor, to periodically participate in the commutation of the motor. Requiring a microprocessor during normal "at-speed" operation of the motor places a real time processing burden on the system microprocessor. The microprocessor is detained from executing other pending tasks and efficiency suffers. For example, where a single microprocessor is used to control various motors, an increased burden from each motor means a lesser ability to control a plurality of motors.
It is an objective of the present invention to provide a DC commutator independent of sampling devices.
It is another objective of the present invention to provide a DC motor commutation system which minimizes the use of undriven motor windings.
It is a further objective of the present invention to provide a DC commutator which is highly efficient.
It is still another object of the present invention to minimize acoustical and electrical noise by avoiding the switching of motor current.
It is yet another object of the present invention to provide for a DC commutation system which minimizes the participation of microprocessor or other controller during at-speed operation.